The present invention relates to an electromigration-proof structure for a multilayer wiring on a semiconductor device, and more particularly to a semiconductor device with multilayer interconnection structure having an interlayer insulating film with excellent surface flatness and a metallic wiring with high resistance to electromigration.
A multilayer wiring structure is extremely effective in semiconductor integrated circuits such as LSI for increasing the level of integration and the packaging density and for decreasing the chip size. In such a case, it is necessary to keep the surface of the interlayer insulating film flat in order to suppress defective step coverage and breaking failure of the upper layer metallic wirings and to obtain a highly reliable multilayer wiring structure.
For these reasons, in the prior art multilayer wiring structures of the above description, organic coating films or the like are used as the interlayer insulating films. In addition, those films are normally used also for passivation films (covering and insulating films). As for a metallic wiring for each layer, Al (aluminum) film has been used to have a good adhesion to the insulating film.
However, in the prior art semiconductor device with multilayer wiring structure as described in the above, the wiring metallic film having Al as the principal constituent has a low resistance to electromigration. Accordingly, these has been a difficulty to use the Al wirings in high speed devices requiring a large current due to a short lifetime of the wirings.
On the other hand, the lifetime of the wirings can be prolonged by using a metallic wiring with Au (gold) as the principal constituent that has a high resistance to electromigration. However, there is another difficulty in putting it to practical use in the conventional structure because of the disadvantage that the adhesion of Au to the interlayer insulating film and the passivation film is poor. For evaluation of the adhesion of a film, it is usual to employ a peeling test which makes use of an adhesive tape. According to this method, adhesion between a film and an underlying material is evaluated in the form of a comparison with the adhesion between the adhesive tape and the film. More specifically, a "block" pattern of squares with side of about 1 mm is incised on the film using a diamond cutter or the like, the adhesive tape is stuck on it and then stripped, and the adhesion is evaluated by counting the number of peeled "blocks" bonded to the tape. For the adhesion performance of the film, it is required in this evaluation method that the number of peeled "blocks" should always be zero. According to the result of the investigation for the combination of Au and an insulating film, the insulating film that can satisfy the standard was only nitride film formed by plasma CVD method. Since, however, the step coverage of this film is extremely poor, it is not possible to apply this film to a flattened multilayer interconnection structure such as is of interest to the present invention.
It is, therefore, a primary object of the present invention to provide a wiring structure having a metallic wiring which can maintain the surface flatness of the interlayer insulating film and the passivation film, and has a high resistance to electromigration.